There's no way that GT5Prologue
production delayed the release of GT5. Maybe by a month or two, if that.
It's analogous to an author working on a book, and their publisher
asking for the first few chapters to review. Prologue represents the first few
chapters of the GT5 book.
Also, what a great way to make money.
I'll bet the book industry wishes they could make money that way.
And the gaming industry no doubt feels the same--make ~ $150 million on
a beta/demo.
I also think it represents a clear cut strategy by Sony/Polyphony.
Release Prologue to make enough money to fund the production
of GTMobile (

) and GT5. Consequently, Sony is not in a rush
to release GT5. They're probably still making around $1 million per month (maybe even more)
on Prologue sales, even at the now-reduced price.
So I think the argument can be made in a roundabout way, that the
fact that GT5Prologue was
released and has generated major cash has, in effect, delayed
the release of GT5. I think without the revenue generated by Prologue
sales, Sony might have been more inclined to put the screws to Polyphony
and get them to release GT5 sooner.
So I for one, would rather not see Prologues released in the future.
They are a way for Sony to make money and justify delaying the release
of the full titles.